As technology advances, the options for communications have become more varied. For example, in the last 30 years in the telecommunications industry, personal communications have evolved from a home having a single rotary dial telephone, to a home having multiple telephone, cable and/or fiber optic lines that accommodate both voice and data. Additionally cellular phones and Wi-Fi have added a mobile element to communications.
Until relatively recently, the primary use of cellular communications has been for voice communications between people. Even for this usage case, considerable optimization has been performed to carry the comparatively small amount of traffic in an optimal way, for instance the introduction of lower bit-rate codecs. In the last few years, the amount of information that needs to be transmitted has begun to increase dramatically with the increasing ubiquity of smart phones, wireless data services and users communicating with machines, particularly servers on the Internet. The next trend will likely be for mobile devices to be used to support machines communicating with other machines (e.g. “M2M”), for instance for remote monitoring and control and many other applications.
As is well known in cellular telecommunications, radiocommunication services are typically provided via an access point, e.g., a base station, which transmits signals to, and receives signals from, mobile devices (also sometimes called user equipment (UEs)). In order to support radiocommunications with a large number of devices which are within its transmission range, a base station will typically transmit and receive signals on a number of different channels and, therefore, will typically include a multi-channel transmitter.
Digital pre-distortion (DPD) is a widely applied technology which is used in the linearization of base stations' multi-channel transmitters. Power efficiency and system performance improvements are achieved by continuously monitoring and improving the output signal in an adaptive, closed loop. The output signal of each transmitter is fed back through a dedicated receiver and down-converted to its digital baseband frequency where a direct comparison with the reference, modem-generated downlink signal can be performed.
For example, as shown in FIG. 1, a traditional DPD system can include multiple transmitter channels or paths 1, . . . , N. Each of the transmitter paths 1, . . . , N includes a transmitter TX1, . . . , TX N and a power amplifier PA1, . . . , PA N. For each of the multiple transmitter paths signals are coupled from the output of a respective PA via couplers 100, 102, 104, etc., and are fed back to a respective feedback receiver 1, . . . , N for radio frequency processing and then on to a digital base band DPD unit 106. However using a separate feedback receiver for each path occupies a lot of space in the base station and consumes significant power.
Another approach is, therefore, to provide a single feedback receiver 200 and switch its processing inputs between the various transmitter channels as shown in FIG. 2 such that the feedback receiver 200 is shared among the various channels and provides feedback for each channel based on switched inputs. More information regarding an example of such a shared feedback receiver can, for example, be found in PCT Application WO 2010/054499.
However, there remains a need for improvement with respect to shared feedback receivers used in digital pre-distortion circuitry associated with multi-channel transmitters.